Conventionally, switching power supplies having a constant current drooping characteristic are used, for example, as power supplies for chargers. That is, the switching power supplies having a constant current drooping characteristic can charge batteries and so on by using the constant current drooping characteristic.
For example, in order to obtain a constant current drooping characteristic in the conventional art, the secondary side of a switching power supply comprises an output current detecting resistor for detecting an output current, a constant current control circuit for controlling current passing through the output current detecting resistor at a constant value, and a photocoupler for transmitting the signal of the constant current control circuit to the primary side. When the output current is equal to or higher than a certain value, the constant current control circuit operates.
However, the constant current control circuit and the photocoupler are expensive and the number of components is increased by using the constant current control circuit and the photocoupler. Thus, the conventional configuration cannot reduce the size and cost of the switching power supply. Further, the conventional configuration causes a power loss in the output current detecting resistor, the constant current control circuit, and the photocoupler, so that energy conservation or high efficiency cannot be achieved.
A method of improving a drooping characteristic is conventionally proposed to prevent excessive output current at overload such as a short circuit. In this method, the drooping characteristic is improved by reducing the detection level of overcurrent protection on the primary side when the output voltage on the secondary side of a switching power supply decreases to one or more predetermined output voltages (e.g., Japanese Patent Laid-Open No. 6-149396).
However, this conventional technique aims to restrict the peak value of output current at overload such as a short circuit by reducing the detection level of overcurrent protection on the primary side according to a decrease in output voltage, so that the switching power supply is safely protected from serious overload such as a short circuit. Hence, the conventional technique cannot control output current so as to have a constant current drooping characteristic when output voltage droops. Therefore, the switching power supply with a drooping characteristic improved by the conventional technique cannot be used as a charger. Moreover, when output voltage is drooped while the peak value of output current is restricted at the detection level of overcurrent protection on the primary side, variations in oscillation frequency and variations in the inductance of a transformer affect the drooping characteristic, resulting in extremely large variations in the overall drooping characteristic. Thus, it is quite difficult to obtain a constant current drooping characteristic.
Another technique for obtaining a constant current drooping characteristic is conventionally proposed in which a constant current drooping characteristic is obtained by reducing an oscillation frequency after current passing through a switching element reaches the detection level of overcurrent protection on the primary side at overload (e.g., Japanese Patent Laid-Open No. 2002-300777).
However, also in this conventional technique, variations in oscillation frequency and variations in the inductance of a transformer affect a drooping characteristic, resulting in extremely large variations in the overall drooping characteristic. Thus, it is quite difficult to obtain a constant current drooping characteristic.